The present invention relates to systems and methods for prosthetics and, more particularly, to systems and method for femoral prosthesis for knee replacement implants and methods of using femoral prosthesis for knee replacement.
Knee replacement procedures, such as total knee arthroplasty (“TKA”) are a highly successful surgical treatment option for severe knee joint diseases, such as osteoarthritis and rheumatoid arthritis. However, several biomechanical and clinical studies have shown that current TKA prostheses do not fully restore the normal function of the knee. Typically, the active range of knee flexion following TKA is less than 115 degrees, whereas the healthy knee is capable of knee flexion up to 160 degrees (Sultan P G et al., Optimizing Flexion After Total Knee Arthroplasty: Advances in Prosthetic Design, Clin Orthop Relat Res. 2003: 167-173; Dennis D A et al., Factors Affecting Flexion After Total Knee Arthroplasty, Clin Orthop Relat Res. 2007; 464:53-60). Even when compared to age-matched control population, TKA patients have been shown to have reduced knee flexion range. For example, Nutton et al. showed that the average active flexion range for TKA patients was 110 degrees (average age 71 years), whereas the age matched control population (average age 69 years) had average active knee flexion range of 134 degrees (Nutton R W et al., A Prospective Randomised Double-blind Study of Functional Outcome and Range of Flexion Following Total Knee Replacement with the NexGen Standard and High Flexion Components, J Bone Joint Surg Br. 2008; 90(1):37-42). Similarly, Noble et al. found that TKA patients had more functional impairment compared to normal subjects of similar age, particularly with regards to deep knee flexion activities involving kneeling and squatting (Noble P C, Gordon M J et al., Does Total Knee Replacement Restore Normal Knee Function, Clin Orthop Relat Res. 2005; 431:157-165).
These types of activities are particularly important for occupations such as roof tiling, leisure activities such gardening, and patients from certain ethnic and cultural backgrounds (e.g. Japan, India, China, and other Asian and Mideastern countries) (Sultan P G et al., Optimizing Flexion After Total knee Arthroplasty: Advances in Prosthetic Design, Clin Orthop Relat Res. 2003: 167-173; Dennis D A et al., Factors Affecting Flexion After Total Knee Arthroplasty, Clin Orthop Relat Res. 2007; 464:53-60). Increased range of knee flexion is also important for meeting the higher demands of younger patients who are increasingly receiving these prostheses. In recongnition of these needs, several major orthopaedic companies have put forth so-called high-flexion TKA prostheses (e.g. Sigma CR150 High Flex from Depuy Inc, and NexGen CR-Flex from Zimmer Inc). However, recent studies have shown that these High-Flexion TKA prostheses offer no advantage over standard prostheses with regards to increasing the range of knee flexion (Gandhi R et al., High-flexion Implants in Primary Total Knee Arthroplasty: a Meta-analysis, Knee, 2009; 16(1):14-7).
One of the causes for restricted knee flexion range with current TKA protheses is that they create excessive tension in the ligamentous structure of the knee in deep flexion. Deep flexion of the knee is generally described as flexion greater than about 115 degrees flexion. This may result from overstuffing of the flexion joint space by the prosthetic components (Varadarajan K M et al., Tibiofemoral Joint Space Measured During Weight-Bearing Knee Flexion Increases Following TKA, Proceedings of 56th Annual Meeting Orthop Res Soc, New Orleans, La., March 2010). In a series of inter-related studies Jeffcote et al., Nicholls et al. and Kuster et al. used a combination of miniature force plates and spring loaded rods to understand the pattern of soft tissue tension in the native knee and in the knee after implantation of contemporary (prior art) TKA prosthesis (Jeffcote B et al., The Variation in Medial and Lateral Collateral Ligament Strain and Tibiofemoral Forces Following Changes in the Flexion and Extension Gaps in Total Knee Replacement, A Laboratory Experiment Using Cadaver Knees, J Bone Joint Surg Br. November 2007; 89(11):1528-33; Nicholls R L et al., Tibiofemoral Force Following Total Knee Arthroplasty: Comparison of Four Prosthesis Designs in Vitro. J Orthop Res. November 2007; 25(11):1506-12; Kuster M S et al, Assessment of Isometricity Before and After Total Knee Arthroplasty: a Cadaver Study. Knee. October 2009; 16(5):352-7). In these studies the soft tissue tension in the native knees was found to be relatively low and uniform in the 15-90 degrees flexion range, such as further illustrated in the graphs of FIGS. 1(A) and 1(B). The native kees were also slightly tighter in full extension (0 degrees flexion), and gradually tightened from 90 degrees to 150 degrees flexion. Here, tightness of the joint implies increased soft tissue tension. Following implantation of contemporary TKA prosthesis, the knees showed a similar pattern of soft tissue tension as the native knees in the 0-90 degrees flexion range, such as indicated in FIGS. 1A and 1B. However, beyond 90 degrees flexion the TKA knees showed a rapid increase in soft tissue tension. This was seen for various contemporary TKA designs particularly those involving the retention of the posterior cruciate ligament (Nicholls R L et al., Tibiofemoral Force Following Total Knee Arthroplasty: Comparison of Four Prosthesis Designs in Vitro, J Orthop Res. November 2007; 25(11): 1506-12). This excessive tightening of the soft tissues of the knee in deep flexion could contribute to the restricted range of knee flexion following implantation of contemporary TKA prosthesis.
The above studies were done on cadaver knees under non-weightbearing conditions. However, these findings are also supported by a more recent study wherein the tibiofemoral joint space in knees of TKA patients was measured during a weight-bearing activity, and compared to tibiofemoral joint space in healthy knees of normal subjects (Varadarajan K M, Yue B., Moynihan A L, Seon J K, Freiberg A A, Rubash H E, Li G., Tibiofemoral Joint Space Measured During Weight-Bearing Knee Flexion Increases Following TKA. Proceedings of 56th Annual Meeting Orthop Res. Soc., New Orleans, La., March 2010). This study found that the TKA knees showed increased tibiofemoral joint space compared to healthy knees for flexion above 90 degrees, such as further illustrated in the graph of FIG. 2. Herein, the tibiofermoral joint space was defined as the distance between a point on the femur bone and a point on the tibia bone measured in the proxima-distal direction. Most TKA patients in this study could not bend their knees beyond 110 degrees flexion. The increased tibiofemoral joint space in flexion may be lead to increased soft tissue tension, which may contribute to restricted range of knee flexion with prior art TKA prosthesis.
Therefore, it would be desirable to have a system and method that can provide deep knee flexion without creating excessive tension in the ligamentous structure of the knee, and thereby to restore native anatomy and function of the knee.